The present invention relates generally to microlithographic mask structures and methods, and more particularly to mask-making for attenuated phase-shift lithography.
A key component of the vast and continuing progress in integrated circuits has been the ability to pattern ever smaller dimensions. These extremely small dimensions are not patterned with visible light. Instead, shorter-wavelength radiation, such as ultraviolet light, x-rays, or electron or ion beams is used in a lithography process. Electron beam and ion beam lithography produce extremely small dimensions, much smaller than is currently required for integrated circuit manufacture, but these methods can only write one pixel at a time. They are unsuitable for rapid volume manufacture.
For transferring a formed pattern onto an integrated circuit, an imaging method is used. The starting point is a xe2x80x9creticle,xe2x80x9d which already contains a magnified version of the pattern that is desired to be reproduced on the chip. In a machine called a xe2x80x9cstepper,xe2x80x9d the reticle is illuminated from behind by a condenser system, typically using ultraviolet light, and the image of the reticle""s pattern is projected by a lens onto a layer of photoresist material on the chip. This optical patterning step is known as xe2x80x9cphotolithographyxe2x80x9d or xe2x80x9cmicrolithography.xe2x80x9d Typically, each exposure of the photosensitive material projects the reticle pattern onto only a portion of the wafer. After each exposure, the stepper xe2x80x9cstepsxe2x80x9d the wafer into a new position for a new exposure. The photoresist may be positive or negative. For a positive photoresist, regions which have electromagnetic radiation applied dissolve upon application of a developer solution, while regions which are not exposed do not dissolve in the developer. With a negative photoresist, the reverse occurs. lithography is so important to semiconductor fabrication that improvements in the resolution of the photoresist patterning are constantly necessary. However, modern stepper systems already operate so close to the fundamental limits of physical law that further improvements are very difficult.
FIG. 1 is a schematic view of a typical main optical train in a modern stepper. Many key elements, such as autofocusing, positioning, vibration control, and cooling, are omitted in this simplified drawing. Light from an ultraviolet source 110 is captured by a condenser system 120 (shown here, for simplicity, as a simple lens) to provide backlighting of a reticle 130. An objective lens system 140 which may contain dozens of lens elements images the reticle""s pattern onto a wafer 150.
The dimensions used are so close to the absolute physical limits of resolution that much of the black/white contrast of the pattern is lost, even when alignment and focusing are perfect, and even if the lens optics were perfect (which, in practice, they never are). The three traces on the left side of FIG. 2 show how a black/white pattern in the reticle is imaged onto the photoresist. (For simplicity, this drawing is dimensioned as if there were no magnification in the stepper lens, although, in practice, the reticle would typically be larger by a factor of 4:1 or so.) Suppose, for example, that the reticle is illuminated by ultraviolet light at a wavelength of 365 nm (xe2x80x9ci-linexe2x80x9d), and the center-to-center spacing of lines in the desired pattern is 500 nm (i.e., lines and spaces are 0.25 micron wide); then, the image will be severely diffraction-limited, as shown.
Trace A on the left side of FIG. 2 shows the distribution of electric field strength (at optical frequencies) at the surface of the reticle. (The mask pattern is assumed to be a conventional one where every portion of the pattern is all opaque or all transparent, i.e., this is a xe2x80x9cbinaryxe2x80x9d mask.) As shown in Trace B, the best imaging optics can image this binary pattern only as a gentle modulation of the electric field at the surface of the photoresist. The incident power density, as shown in Trace C, is proportional to the square of the electric field. The square of a function is always sharper than the function itself. Image contrast is limited even though the power density is proportional to the square of the electric field. Thus, the black/white pattern of the binary reticle is imaged as merely a gray-on-gray pattern on the photoresist.
Conventionally, photoresist chemistry is optimized to discriminate between the brighter and darker intensities. FIG. 3 shows a typical photoresist response profile. Photoresist locations which have received an integrated dose (per unit area) of E1 or less will be unaffected, but a dose of E2 or more will cause the photoresist to be cleared. The slope of this curve at doses below E2 is an important measure of sensitivity.
One way to push the limits of resolution farther is to use interference techniques to increase the range of intensities imaged onto the surface of the photoresist. There are several ways to do this One preferred way is Attenuated Phase-Shift Lithography, or APSL. APSL techniques have two notable differences from conventional binary masks. First, the masking material is not fully opaque but is made slightly transmissive. Second, the thickness of the clear substrate, in the bare areas between the lines of the masking material, is adjusted by selective etching to create a phase shift of about 180 degrees, at the primary wavelength used for imaging, with respect to the light which passes through the masking material. This phase shift creates destructive interference: light which diffracts from the open areas into the dark areas will interfere destructively with light passing through the masking material, so that the intensity will actually pass through zero between a bright area and a dark area.
The three traces on the right side of FIG. 2 show how phase-shifting methods achieve an improvement over the conventional binary mask. Trace D in FIG. 2 shows the distribution of electric field strength at the surface of the APSL reticle. The field strength is negative, rather than zero, at the dark parts of the reticle. This is due to the phase reversal between the lines and spaces of the reticle. Trace E shows how the same imaging optics would translate the field distribution into a field distribution at the surface of the photoresist. Just as in Trace B, the distribution seen in Trace E has been smoothed out greatly by the imaging optics. However, Trace E, unlike Trace B, includes negative portions. Even though the negative portions of Trace E are narrower than the negative portions of Trace D, the field strength passes through zero near each edge, and the power density of Trace F also hits zero near each edge. As may be seen by comparing Traces C and F, this results in a great improvement in contrast.
Every type of phase-shift masking has its limitations. For example, phase-shift lithography can be particularly useful in areas which have very regular layouts, such as arrays or subarrays of memory cells, since the pattern can be modified as needed to achieve the best possible imaging performance. However, locations which have random layout, or which have very sparse images, may do better with non-phase-shift lithography.
Phase shift reticles produce images with improved resolution and depth of focus primarily for patterns that are repeating as one finds in an array. To accomplish this, the stepper illumination conditions (numerical apertures, off-axis, on-axis, quadrupole, and the like) are optimized to provide the best depth of focus and resolution for the array. Outside the array periphery, the pattern is not repeating and the resolution or depth of focus in the periphery can be degraded due to the phase shift reticle. So there is a need to develop a simple, manufacturable reticle technology that produces a phase shift patterning capability in one part of the reticle but yet produces a non-phase shift patterning capability in another part of the reticle.
The present application, in various embodiments, discloses a photolithography masking technology in which a single semi-opaque patterned layer is used to provide both phase-shift and conventional lithography on the same integrated circuit. This is accomplished by patterning the transmissive substrate so that in some areas, the etched substrate pattern provides a phase-shift which creates destructive interference relations, and in other areas, the etched substrate pattern creates a phase-shift which does not provide these destructive interference relations.
In one embodiment, a method of forming a lithographic mask includes patterning a mask layer having a periphery pattern and an array pattern on a substrate, etching the periphery pattern and the array pattern to create a first region with zero phase shift between the mask layer and the substrate, and etching the array pattern to create a second region with a different phase shift between the mask layer and the substrate.
In another embodiment, a photolithographic mask includes a transmissive substrate having a first region and a second region and a mask layer etched from a mask material in a fine line pattern. The substrate is recessed in the first region to a first depth and recessed in the second region to a second depth different from the first depth to provide no phase shifting between the mask layer and the substrate in the first region and to provide non-zero phase shifting between the mask layer and the substrate in the second region.
Other embodiments are described and claimed.